1. Field of the Invention
The present invention relates to a polysulfone separation membrane which exhibits a high oxygen rate and high oxygen nitrogen selectivity not found in the prior art, and a superior heat resistance and chemical resistance, and a method of preparing same. The polysulfone asymmetric membrane, e.g., polysulfone asymmetric hollow fiber membrane comprising the polysulfone as the base material, has a high oxygen nitrogen selectivity and can be used for obtaining oxygen-enriched air having an oxygen concentration of 40%, which can be used for medical and biological uses, or nitrogen-enriched air having a nitrogen concentration of 95% or higher, which can be used for food storage and to prevent explosions in an organic solvent tank. Further, this membrane can be used for removing carbon dioxide from a combustion furnace and from natural gas.
2. Description of the Related Art
The utilization of nitrogen-enriched air with a nitrogen concentration of 95% to 99.5% has long attracted attention, and as an example of the use of this nitrogen-enriched air, an attempt has been made to prevent explosions by filling a combustible liquid storage tank with the nitrogen-enriched air, since this is an inert gas. The nitrogen-enriched air is produced directly from the atmosphere by a membrane separation technique, and when obtaining nitrogen-enriched air by this membrane separation technique, non-permeated gas having a lower oxygen concentration can be used by increasing the amount of gas permeated through the membrane, by using an oxygen permselective membrane. In this case, if the membrane has a high oxygen/nitrogen selectivity, a nitrogen-enriched air having a desired nitrogen concentration can be efficiently produced while using less energy. A large volume of gas must be treated to obtain nitrogen-enriched air on an industrial scale by the membrane separation technique, and thus a membrane having a higher oxygen permeation rate is required. Nevertheless, among presently commercially available oxygen enrichment membranes, a membrane having a combination of a high oxygen/nitrogen selectivity and a high oxygen permeation rate is not known, and the production of a nitrogen-enriched air utilizing membrane is limited.
The production of a gas separation membrane having a high oxygen permeation rate and high oxygen nitrogen selectivity has become possible due to the development of a polymeric material and the development of a separation membrane preparation method. The aim of this development is to develop a base material which (1) has a high oxygen permeation coefficient, (2) has a high oxygen/nitrogen selectivity, (3) is capable of being formed into a thin film and (4) can withstand a high temperature gas. In the development of a polymeric material with a high oxygen permeation coefficient, for example, the development of a polydimethylsiloxane type material is known (see, for example, PB Rep. No. PB-85-148476, but this membrane is formed of a dimethylsiloxane type material and has a limited oxygen/nitrogen selectivity and a poor heat resistance, and thus cannot be used in a high temperature process.
The utilization of a separation membrane for a high temperature process has been investigated, and the development of membrane materials having a good heat resistance is underway. For example, polysulfone obtained from a sodium salt of bisphenol A and 4,4'-dichlorodiphenylsulfone is known as a membrane base material having a good heat resistance, but the glass transition temperature (Tg) thereof (180.degree. C.) is not enough, at 180.degree. C., and a further improvement of the heat resistance thereof is required.
As a method of improving the heat resistance, the introduction of a bisphenylfluorene group is known; for example, a polysulfone obtained from 9,9-bis(4-hyiroxyphenyl)fluorene and dichlorodiphenylsulfone has a glass temperature (Tg) of 280.degree. C. A method of synthesizing a polysulfone containing a bisphenylfluorene group has been reported by Hergenrother et al, (P. M. Hergenrother et al, Polymer, vol 29, p. 358-369, 1988), but according to this method the polysulfone obtained from 9,9-bis(4-hydroxyphenyl)fluorene and dichlorodiphenylsulfone has only an inherent viscosity (.eta..sub.inh) of about 0.67 dl/g (in m-cresol, measured in a solution containing 0.5% of polysulfone dissolved therein at 30.degree. C.), and an improvement of the degree of polymerization is required to improve the strength of the polysulfone molded product.
As the thickness of the gas selective functional layer and the permeation rate per membrane unit area have an inversely proportional relationship, the aim of the development target of the separation membrane preparation method is to form the gas selective functional layer as a thin layer. Further, it is important to obtain a hollow fiber membrane formation, to increase the membrane area per module unit volume area. As a prior art method by which these problems are solved, it has been proposed to form a composite of a thin film having a required functional ability, and a supporting porous film. For example, an attempt has been made to prepare a very thin gas separation layer of about 30 nm by adding the polycarbonate-polydimethylsiloxane block copolymer solution dropwise onto the surface of a liquid cast support, to thereby form a composite with a microporous flat film, such as a Millipore ultrafiltration film, etc. (See Japanese Unexamined Patent Publication (Kokai) No. 54-40868 or U.S. Pat. No. 4132824). Nevertheless, the preparation of a composite membrane having such an extremely thin gas selective functional layer on the surface, without defects such as pinholes or cracks, is difficult in practice.
A more preferable method of forming a thin gas selective active layer is to produce an asymmetric skinned membrane having a structure in which a thin layer on the membrane surface, called a skin layer, is supported on a porous layer, called a finger layer, and a sponge layer, by dissolving a polymer in an appropriate solvent and desolventizing the solution by contact with a coagulation liquid which is miscible with the solvent but does not dissolve the polymer (see, for example, C. A. Smolders et al., "Hollow Fiber Gas Separation Membranes; Structure and Properties", p. 145 in "Membranes in Gas Separation and Enrichment, 4th Boc. Priestley Conference, Royal Society, 1986). This method has specific features such that the thin layer can be formed without the need for the cumbersome operation of forming a composite, and that a hollow fiber membrane can be easily obtained. Nevertheless, in this method the polymer must be soluble in the solvent, and a suitable combination of a polymer/polymer solvent/ coagulation liquid must be obtained. If these conditions are not satisfied, even if an asymmetric structure is obtained, either no gas selectivity is exhibited or such a selectivity, if any, is usually lower than that of a uniform film. As an example of the asymmetric skinned gas separation membrane, a separation membrane having an oxygen permeation rate of 3.53.times.10.sup.-3 Nm.sup.3 /m.sup.2 .multidot.hr.multidot.atm and an oxygen/nitrogen selectivity of 3.7 is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 58-8504, and a separation membrane having an oxygen permeation rate of 3.2.times.10.sup.-2 Nm.sup.3 /m.sup.2 .multidot.hr.multidot.atm and an oxygen/nitrogen selectivity of 4.8 is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-90709. These asymmetric skinned membranes, however, have a low oxygen permeation rate and oxygen nitrogen selectivity, and an improvement of those characteristics is required.